The present invention pertains to a navigation system employing signals in the LF/MF range, and particularly to such navigation system wherein LF/MF transmitters are frequency stabilized to allow navigation by phase differential techniques.
Presently, RDF (Radio Direction Finder) receivers are used in coastal water craft, or other vehicles, to provide an inexpensive means for determining position. An RDF receiver is capable of determining the bearings from the craft to geographically separated radiobeacon transmitters, and the position of the craft may be determined by plotting the intersections of the back azimuths of the bearings on an accurate chart, each of the back azimuths comprising LOP (Line of Position).
For certain applications, for example, for navigating through a narrow strait or to locate a fishing hole, a water craft may require a more accurate means of navigating than is provided by an RDF receiver but which is still relatively inexpensive. Such means could comprise a receiver capable of receiving a signal from each of two radiobeacon transmitters and of indicating which hyperbolic line of the hyperbolic phase grid constructed by the two signals the craft is on by computing their phase difference. If the signals have different frequencies, they must be translated to a common frequency f in order to determine the difference between their phase angles. Navigation systems based on hyperbolic phase grids are of course well-known for frequencies in the VLF range (3-30 kilohertz). However, present radiobeacon transmitters transmit in the LF/MF range (250 kilohertz-490 kilohertz) with relatively low frequency stability. Consequently, the differential frequency error, .DELTA.f/f, the difference between the actual translated values of the transmitted signals, may exceed 10.sup.-5. With an error of this magnitude, positional error will accumulate at the rate of one mile/second, which may prevent the system from providing useful accuracy.